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Pomological Tests 



A. Comparison of virus-free and virus -infected trees 



At the Wilhelminadorp Research Station and the 6 experimental 

 gardens in the Netherlands, trials are established to compare the 

 "old" virus-infected trees, the virus-tested trees, and "the new" 

 virus-free trees. So far only preliminary results have been ob- 

 tained (van Oosten 1974 and 1975), but these seem to confirm, in 

 general, the older data of the English research stations at Long 

 Ashton and East Mailing in that (a)virus-free trees grow more vig- 

 orously than virus- infected trees; (b)virus-free trees usually pro- 

 duce larger yields than virus-infected trees; (c)the yield effi- 

 ciency (pounds per unit of growth) of virus-free trees is equal or 

 higher than of virus-infected trees and Cd)fruit quality (color, 

 smoothness of fruit skin, grade) is also usually better for virus- 

 free trees. 



These conclusions underline important advantages of virus-free 

 trees. But in the Netherlands, the stronger growth of virus-free 

 trees is not always considered as positive. The planting system 

 in the Netherlands is based on small trees on weak (virus-infected) 

 M.9 rootstock with tree spacing of 10.7 or 11.6 feet between the 

 row and 4.1 or 4.9 feet in the rows, depending on the cultivar. 

 Tree height does not exceed 8.2 to 9.8 feet to permit picking with- 

 out ladders. The stronger growth of virus-free trees will affect 

 tree-spacing, tree height, and perhaps increase pruning time. 



The Dutch experiments have shown that on apple replant soil * 

 the stronger growth of virus-free trees seems to be acceptable in 

 the first years after planting (Table 1). The growth of normal 

 virus-infected trees in replant soil is often considered to be too 

 weak in the first years after planting. It is questioned how 

 growth of virus-free trees will develop later. In experiments on 

 new land for apple , the growth is considered too strong, especially 

 for some strong-growing cultivars (Table 1) . It is expected that 

 on new land planting distances may have to be greater. It is ques- 

 tioned what the influence will be on tree height in later years and 

 what the consequences are for yield per acre. The growth of virus- 

 free trees especially on fresh apple soil will be one of the main 

 concerns of Dutch fruit research in the next years. 



B. Testing of tip-grafts 



In the trials mentioned above, only a very limited number of 

 tip-grafts of a few cultivars is included. Most of the produced 

 virus-free tip-grafts are therefore studied in other trials at the 

 Wilhelminadorp Research Station and the Experimental Gardens. This 

 is to be sure that no unexpected mutations have occurred and to 



*Soil previously planted to apples. Trees grow less vigorously in 

 replanted soil than in "new" soil or in fumigated replant soil. 



